Concerning ATC providing pressure reading: Below the transition altitude (the altitude where you go from HG to SPS [standard pressure setting]) aircraft work on HG. Once they reach the transition altitude, which varies in different countries and can be found on the relevant charts, they switch to SPS, which is 1013.25MB, or 29.92 Inches of Mercury. These are then called flight levels. The main purpose of having flight levels are to maintain a set vertical distance between aircraft. That's why it's important to have SPS set when you need it set, if one aircraft is going head on towards another and they have different altimeter settings, they could collide. I can think of a couple of incidents where that has happened.
Basically, pressure can vary. A mass of colder air, for example, increases the pressure lapse rate and causes isobars to dip. A mass of warmer air causes the pressure lapse rate to decrease, making isobars expand. (Isobars - lines joining places of equal pressure). Therefore, the 600hpa level in a mass of cold air is lower than it is in a mass of warm air. The basic rule is, "From high to low, look out below", which means that when you're flying from an area of high temperature or pressure to an area of low temperature or pressure, your altimeter is overreading, and your true altitude is lower than what's being indicated to you. But, if all aircraft have SPS (1013.25, or 29.92) set they will all maintain their separation.
Every half hour the ATIS is updated and the HG will be given. Once you're above transition altitude, you set SPS and forget about it. On the way back down, when you pass transition level (where you go from SPS back to HG) you set the regional HG (lowest forecast HG for the area you're flying in) or the HG for your aerodrome.
Here in the u.s. its 18,000 ft msl. above that its just set 29.92 and forget about it.
I dont think this is modeled in DCS but I may be wrong.